The present invention relates to a water impervious heat shrinkable tube which is impermeable to water.
A heat shrinkable rubber or plastic tube is obtained by, for example, a method wherein a rubber or plastic such as polyethylene, polyvinyl chloride, polyester, or ethylene propylene rubber is melted and extruded in a tubular form, the obtained tube is irradiated with an electron beam to be crosslinked; and the crosslinked tube is stretched at a temperature which is lower than the melting point of the rubber or plastic used and higher than the softening point thereof. According to another method, a sheet of a rubber or plastic as mentioned above is irradiated with an electron beam to be crosslinked, stretched by a desired multiple of its initial length, wound to a desired thickness around a mandrel having a desired diameter, and then made into an integral tubular form by heating at a temperature higher than the melting point of the rubber or plastic used.
A heat shrinkable rubber or plastic tube prepared in this manner (to be referred to as a heat shrinkable tube hereinafter) is capable of returning, when heated, to its original shape seen prior to stretching. Utilizing this property, a tight rubber or plastic covering may be formed on a desired object. For this reason, heat shrinkable tubes have been recently used in various fields.
However, when a heat shrinkable tube is used in water for a long period of time, water permeate through the rubber or plastic covering. Therefore, such a heat shrinkable tube may not be used where a high water impermeability is required.
Meanwhile, power cables are required to be water impermeable in order to prevent insulation degradation due to formation of a water-tree in an insulator which is caused by water permeation into the cables. In order to form a protective layer at a joint of power cables, a method is generally adopted wherein a heat shrinkable rubber or plastic tube is fitted around a cable joint and is heated to be shrunk. However, for the same reason as described above, the joint thus prepared does not have satisfactory water impermeability.
As a means for improving the water impermeability of a heat shrinkable tube, the present inventors have previously proposed a heat shrinkable tube wherein a metal foil film which is almost completely impervious to water is so laminated on the inner surface of the tube that the metal foil film extends along the entire circumference of the tube.
The heat shrinkable tube previously proposed by the present inventors has the configuration shown in FIGS. 1 and 2. Referring to FIGS. 1 and 2, two or more metal foil films 2 extend along the entire circumference of the tube with their longitudinal edges 2a overlapped each other in a predetermined width to follow the shrinkage of a heat shrinkable material layer 1. However, a water impervious heat shrinkable tube of this type has a large number of lapped portions, resulting in a complex configuration and a complex manufacturing process. In addition to this, depending upon the positions of the metal foil films 2 formed in the heat shrinkable tube 1, the metal foil films 2 form rough wrinkles which make gaps in the longitudinal direction of the tube 1 when the tube 1 is shrunk. Thus, the metal foil films 2 may not be tightly fitted on the cable joint.
In an extreme case, as shown in FIG. 3, the metal foil films 2 form rough wrinkles 3 in the longitudinal direction of the tube 1 upon shrinkage thereof. This results in the formation of small gaps 5 which extend on the outer circumferential surface of a cable 4 in the longitudinal direction thereof. Then, a problem arises in that when water permeation is caused from the ends of the coated portion of the cable or from a damaged portion thereof, water thus introduced permeates along the longitudinal direction of the cable. In addition, when such wrinkles are formed, the outer surface of the cable becomes nonuniform, providing a poor outer appearance.